N-containing plasma etch process with reduced resist poisoning

ABSTRACT

A method for forming a metal interconnect comprises exposing a dielectric layer to an etch chemistry containing nitrogen-containing compound such as NH 3 , NF 3  or N 2 O. The nitrogen-containing compound provides selectivity and/or profile control comparable to that provided by N 2 , while avoiding poisoning of photoresist by embedded N 2 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to Ser. No. 09/887,166, filed Jun. 25, 2001,and to Ser. No. 09/887,084, filed Jun. 25, 2001, and to Ser. No.09/886,989, filed Jun. 25, 2001.

FIELD OF THE INVENTION

The present invention relates to photolithographic etching of asubstrate, and in particular to a method for preventing poisoning of aphotoresist during semiconductor fabrication.

BACKGROUND OF THE INVENTION

Photolithographic techniques are commonly used in the fabrication ofintegrated circuits. Photolithography entails coating a surface of asubstrate that is to be etched with a photoresist, which is thenselectively exposed to electromagnetic radiation, for example using areticle to define a selective exposure pattern on the photoresist, anddeveloped to define a pattern in the photoresist, which is then used asan etch mask. The patterned photoresist material is removed from thesubstrate after it has been used as an etch mask.

Photoresists are classified according to their response toelectromagnetic radiation. Positive photoresists are applied tosubstrate surfaces as polymers, selected portions of which depolymerizewhen exposed to electromagnetic radiation, for instance using a reticleto define the selective exposure pattern. The depolymerized portions arethen removed from the substrate surface by a developing treatment, suchas by exposure to a developing solvent that selectively dissolvesdepolymerized photoresist. The pattern formed by the photoresist matchesthe reticle pattern. Negative photoresists are applied as monomers orlow molecular weight polymers, which polymerize or cross-link uponexposure to electromagnetic radiation. In the case of a negativephotoresist, the unexposed portions of the selectively exposedphotoresist are removed during development. The pattern formed byexposure and development is the negative image of the reticle. Hence thedesignation as a negative photoresist. Photoresists that selectivelyreact (i.e. depolymerize, polymerize or cross-link) when exposed to DUV(i.e. ultraviolet light having a wavelength of less than about 300 nm)are referred to as DUV photoresists.

Photolithographic techniques are applicable to selective etching of manydifferent materials. For instance, etching of a dielectric is commonlyachieved by dry etching of a dielectric using a patterned photoresist asan etch mask. Dry etching methods include plasma etching (PE) andreactive ion etching (RIE). In both plasma etching and RIE there is thepotential chemicals in the etch chemistry to poison later-depositedphotoresists. It is common to use N₂ in dry etching to provideselectivity control and profile control. The amount of N₂ is adjusted inrelation to the etchant to provide optimal control of selectivity andprofile characteristics. However, in such etching processes, N₂ maybecome embedded in dielectric layers. Such N₂ may then diffuse into alater-deposited photoresist layer, where it can interfere with thephotochemistry of the photoresist. Thus, N₂ introduced during etchingcan poison photoresist layers, and thereby interfere with laterprocessing steps.

Modern integrated circuit fabrication commonly requires two or morephotolithographic steps. For instance, in a so-called dual damasceneprocess, two photolithographic steps are used to define a hole (via) anda trench. A first photolithographic step is carried out to form a holeor via through a dielectric layer, such as an SiO₂ or an SiON layer, toa conductive layer beneath the dielectric layer. A secondphotolithographic step is then carried out to form a trench connectingto the via. In such a process, a first photoresist is applied to thesurface of a dielectric layer, and the photoresist is then selectivelyexposed and developed to form a via pattern. The via is then formed byselectively etching through the dielectric to the underlying conductivelayer by, for instance, anisotropic dry etching or reactive ion etching(RIE) in a vertical direction. Once the via is formed, the firstphotoresist layer is removed, typically by one or more ash steps. Asecond photoresist layer is then applied, exposed, and developed to forma trench pattern. This is followed by a second etch step, using thesecond photoresist as a mask. The second photoresist layer is thenremoved, the via and trench are filled with a conductive material, suchas a metal or a silicide, and then the conductive material is planarizedback to the level of the dielectric surface.

In the dual damascene process described above, it is essential to removeall exposed photoresist (in the case of a positive photoresist) orunexposed photoresist (in the case of a negative photoresist) during thedeveloping step of each stage of photolithographic processing, becauseresidual photoresist acts as an etch mask. Contaminants in thedielectric may poison photoresists, resulting in defective etching ofthe substrate. While typical clean room conditions generally ensure thatthe first photoresist layer will experience little interference fromexogenous chemical contaminants, second and subsequent photoresistlayers may be contaminated by a number of chemical contaminants(photoresist poisons), such as N₂ embedded in the substrate to beetched. Photoresist poisons tend to block the electromagneticradiation-induced reaction (depolymerization, polymerization orcrosslinking) in the selectively exposed photoresist, resulting inincomplete developing of the photoresist pattern.

To illustrate the problem, a typical prior art process for forming a viaand trench combination is depicted in FIGS. 1A-1H. The prior art processis illustrated for the case where the photoresist layers are bothpositive photoresists, however the skilled artisan will recognize thatthis is merely illustrative and that the same principles apply tonegative photoresists.

A precursor 10 comprising a conductive layer 12 and a dielectric layer14 is depicted in FIG. 1A. A person skilled in the art will recognizethat the precursor 10 can include other features that are not shownbecause they are not essential to understanding the prior art process.Such other features include, for instance, MOS devices, resistors,capacitors, etc. The conductive layer 12 may be, for instance, aluminum,copper, metal silicide, or other suitable conductor. The dielectriclayer 14 may include, for instance, SiO₂ or a low K dielectric material.

As depicted in FIG. 1B, a first photoresist layer 16, is applied overthe dielectric layer 14 by an art recognized method, such as by aspin-on technique. The photoresist layer 16 is then selectively exposedand developed to form a patterned photoresist layer 16, as depicted inFIG. 1C. Next, the portion of dielectric layer 14 that is exposedthrough the first photoresist layer 16 is etched, for instance byanisotropic dry etching in the vertical direction, to partially exposeconductive layer 12, as depicted in FIG. 1D. Photoresist poison 18 isembedded in the dielectric layer 14 during the etching process. Thesource of photoresist poison 18 in this regard is N₂ that has been addedto the etch chemistry to control profile and selectivity.

Then, the photoresist layer is removed by, for instance, a plasma ash,thereby producing the etched dielectric 14 depicted in FIG. 1E. Thearticle 10 comprises conductive layer 12 and etched dielectric layer 14.

FIG. 1F shows the precursor 10 after a second photoresist layer 20 hasbeen deposited. Nitrogen (N₂) 22 in photoresist poison 18 diffuses intothe second photoresist layer 20. The N₂ 22 interferes with thephotochemistry of the second photoresist layer 20 during selectiveexposure, preventing depolymerization of the second photoresist 20.

FIG. 1G shows precursor 10 after selective exposure of the secondphotoresist 20. Residual poisoned photoresist 24 is the result ofincomplete depolymerization of the photoresist 20. This poisonedphotoresist 24 is not removed during developing, and as a result masksportions of dielectric 14 that are intended to be etched during trenchformation.

FIG. 1H shows the precursor 10 after a trench etch step. The poisonedphotoresist 24 has masked portions of dielectric layer 18, resulting inhumps 26. These humps 26 are device defects that may result inincomplete, or no, contact between via and trench conductor materials.Such defects introduced during the photolithographic processing ofintegrated circuits lead to decreased device speed, or in some casesinoperability of the integrated circuit.

There is therefore a need for a photolithographic process that reducesdefects caused by poisoning of photoresists by residual N₂ introducedduring a dielectric etch step.

There is also a need for a photolithographic process that provides anetch step having an etch chemistry that provides selectivity and profilecontrol comparable to that provided by N₂, but that prevents, avoids orameliorates photoresist poisoning caused by N₂ in the etch chemistry.

SUMMARY OF THE INVENTION

These and other needs are met by the present invention, which provides amethod of etching a dielectric layer, comprising the steps of: providingan article having a dielectric layer thereon; depositing a photoresiston the dielectric layer; patterning the photoresist; and exposing thedielectric to an etch chemistry comprising an etchant and one or moremember selected from NH₃, NF₃ and N₂O in an amount sufficient to controlselectivity, profile, or both.

These and other needs are further met by the present invention, whichprovides a method of forming an interconnect structure having a firstfeature and a second feature, comprising the steps of: providing anarticle comprising a conductive layer that has a dielectric layerthereon; depositing a first photoresist layer on the dielectric layer;patterning the first photoresist layer to form a first mask patterndefining the first feature; etching the dielectric layer using the firstphotoresist layer as a mask to form the first feature using an etchchemistry that comprises a member of the group NH₃, NF₃ and N₂O;removing the first photoresist; depositing a second photoresist layer onthe article; patterning the second photoresist layer to form a secondmask pattern defining the second feature; etching the dielectric layerusing the second photoresist layer as a mask to form the second feature.

These and other needs are further met by the present invention, whichprovides a dual damascene method of forming an interconnect structurehaving a via and a trench, comprising the steps of: providing asemiconductor wafer comprising a conductive layer that has a dielectriclayer thereon; depositing a first photoresist layer on the dielectriclayer; patterning the first photoresist layer to form a first maskpattern defining the via; etching the dielectric layer using the firstphotoresist layer as a mask to form the via using an etch chemistry thatcomprises a member of the group NH₃, NF₃ and N₂O; removing the firstphotoresist; depositing a second photoresist layer on the article;patterning the second photoresist layer to form a second mask patterndefining trench; etching the dielectric layer using the secondphotoresist layer as a mask to form the trench; depositing copper or acopper alloy in the trench and via; and etching the copper or copperalloy back to the surface of the dielectric layer.

The foregoing and other features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention, when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1H depict a prior art photolithography process, in whichphotoresist poison prevents removal of a portion of exposed photoresist.

FIGS. 2A-2H depict a photolithography process including an etch stepcomprising a non-N₂ nitrogen-containing compound according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an improved process for etching asubstrate, wherein deposition of N₂, and concomitant photoresistpoisoning thereby is prevented by employing a non-N₂,nitrogen-containing compound in the dielectric etch chemistry. Inparticular, the present invention employs one or more non-N₂,nitrogen-containing compounds such NH₃, NF₃ or N₂O in place of N₂. Thenon-N₂, nitrogen-containing compounds offer etch benefits similar tothose provided by N₂, while reducing or eliminating photoresistpoisoning associated with N₂. The process according to the presentinvention alleviates the problem of nitrogen poisoning of photoresistsand therefore provides for more precise etching of semiconductorfeatures, such as vias and trenches.

While not wishing to be bound by any theory, the present inventorsbelieve that inclusion of a non-N₂, nitrogen-containing compound in theplasma etch chemistry prevents deposition or embedding ofnitrogen-containing chemical species in the dielectric, because non-N₂,nitrogen containing compounds such as NH₃, NF₃ and N₂O do not becomeembedded in the dielectric material. Thus, the non-N₂,nitrogen-containing compounds according to the present invention are notavailable to leach into, and poison, subsequently employed photoresistmaterial. Thus, N₂ poisoning of photoresists is prevented by employingnon-N₂, nitrogen containing compounds such as NH₃, NF₃ or N₂O in thedielectric etch chemistry in accordance with the present invention.

Particular reference is made below to a process according to the presentinvention that is used as part of a method of forming a via and a trenchin a dielectric layer. However, the skilled artisan will recognize thatthe dielectric etch method according to the present invention may beused in a variety of situations in which it is desirable to etch adielectric, and where it is undesirable for N₂ to become embedded in thedielectric surface.

The dielectric etch method according to the present inventionadvantageously comprises one or more steps in which a chemical speciesis brought into contact with the article to selectively etch exposedportions of the article, using a patterned photoresist as an etch mask.In some embodiments according to the present invention, the etch methodcomprises a radio frequency (RF) plasma etch, wherein the plasmachemistry comprises one or more of NH₃, NF₃ or N₂O as agents foraffecting profile or selectivity control. In other embodiments accordingto the present invention, the etch method comprises a reactive ion etch(RIE) ash step, wherein the RIE chemistry comprises one or more of NH₃,NF₃ or N₂O as agents for affecting profile or selectivity control. Instill further embodiments according to the present invention, the etchmethod comprises both a RIE etch step and a RF etch step.

The method according to the present invention may be more fullyunderstood with reference to FIGS. 2A-2H, as described below.

As depicted in FIG. 2A, an article 30 having a metal interconnectstructure is provided, which comprises conductive layer 32 anddielectric layer 34. Dielectric layer 34 is located over conductivelayer 32. In particular embodiments according to the present invention,dielectric layer 34 is adjacent conductive layer 32. In otherembodiments according to the present invention, an anti-reflectivecoating (not shown) may be sandwiched between dielectric layer 34 andconductive layer 32. While not shown, it is to be understood thatarticle 30 advantageously further comprises semiconductor devices suchas NMOS, PMOS and CMOS devices, as well as resistors, capacitors, andother components known to the person skilled in VLSI and ULSIfabrication technology.

The conductive layer 32 advantageously comprises a conventionalconductive material, such as a metal or a metal silicide. Exemplaryconductive metals include aluminum, an aluminum alloy, platinum, silverand copper, while exemplary conductive metal silicides include cobaltsilicide, titanium silicide and nickel silicide. Other conductivematerials are advantageously employed in the method according to thepresent invention. The conductive layer 32 is advantageously formed byan art recognized method, such as conformal deposition, sputtering orCVD, followed by planarization, in the case where the conductive layeris a metal, or by a suitable silicidation method, in the case where theconductive layer 32 comprises a metal silicide.

In some embodiments according to the present invention, the dielectriclayer 34 is a layer of dielectric material, such as SiO₂. In particularembodiments according to the present invention, the dielectric layer 34is a SiO₂ layer that is deposited by TEOS on the conductive layer 32. Inother embodiments, the dielectric layer 34 is a low K dielectric layer.

A first photoresist layer 36 is deposited on the dielectric layer 34 asdepicted in FIG. 2B. The first photoresist layer 36 advantageouslycomprises a conventional photoresist compound, such as a positive DUVphotoresist. The photoresist layer 36 is advantageously applied to thedielectric layer 34 directly by an art-recognized method, such as aspin-on method. In other embodiments according to the present invention,an intermediate layer (not shown) is applied to the dielectric layer 34,after which the dielectric layer 34 is applied to the intermediate layer(not shown), such that the intermediate layer (not shown) is sandwichedbetween the dielectric layer 34 and the photoresist layer 36. Thisintermediate layer (not shown) advantageously includes ananti-reflective coating, which absorbs and dissipates the photon energythat passes through the photoresist, thereby preventing the formation ofstanding waves in the exposed photoresist.

The photoresist layer 36 is then patterned as depicted in FIG. 2C. Thephotoresist layer 36 is patterned by selectively exposing thephotoresist layer 36 to electromagnetic radiation, such as DUV light,and then developing the photoresist layer 36 by removing the selectivelyexposed photoresist. The selectively exposed (positive) photoresist isadvantageously removed by an art-recognized method, such as RF plasmaetch, RIE, or wet chemical dissolution. After developing, thephotoresist layer 36 includes aperture 38.

The dielectric layer 34 is then selectively etched as depicted in FIG.2D. The dielectric layer 34 is selectively etched by subjecting article30 to an etch, such as an anisotropic dry etch, that is selective forthe dielectric. The photoresist layer 36 acts as an etch mask, whiledielectric 34 is etched by an etchant, such as a fluorocarbon, ahydrofluorocarbon, a chlorocarbon, or a fluorochlorocarbon, throughaperture 38. The unmasked portions of dielectric layer 34 are thusselectively etched back to the surface of conductive layer 32 to producevia 42.

The etch chemistry further contains one or more non-N₂,nitrogen-containing compound selected from NH₃, NF₃ and N₂O. The non-N₂,nitrogen containing compound selected from NH₃, NF₃ and N₂O providesprofile and/or selectivity control similar to that provided by N₂, whileat the same time avoiding the deposition of N₂ in the dielectric layer.As discussed above, the etch chemistry comprises an etchant, such as aconventional dry etchant, and a non-N₂, nitrogen-containing compoundselected from NH₃, NF₃ and N₂O. In some embodiments according to thepresent invention, the etch chemistry also comprises one or more inertgases such as He, Ne or Ar.

The etch method according to the present invention uses an etchchemistry that is selective for a particular substrate, such as adielectric. In addition to the non-N₂, nitrogen-containing compoundstaught above, the etch chemistry comprises one or more etchants such asa hydrochlorocarbon, a fluorocarbon, a hydrofluorocarbon, achlorofluorocarbon, etc. In some embodiments according to the presentinvention, the etchant comprises HCF₃.

In some embodiments according to the present invention, the etchchemistry comprises only one non-N₂, nitrogen-containing compoundselected from NH₃, NF₃ and N₂O. In other embodiments according to thepresent invention, the etch chemistry comprises two members selectedfrom NH₃, NF₃ and N₂O. In still further embodiments according to thepresent invention, the etch chemistry comprises all three of NH₃, NF₃and N₂O.

In some embodiments according to the present invention, the etchchemistry also comprises one or more inert carrier gas, such as helium(He), neon (Ne), or argon (Ar).

In embodiments according to the present invention where the etchchemistry includes more than one member selected from NH₃, NF₃ and N₂O,the mole ratio of one member to another is advantageously selected tomaximize profile and/or selectivity control. For instance, inembodiments according to the present invention, wherein the etchchemistry includes two members selected from NH₃, NF₃ and N₂O, the moleratio of the first member to the second member is advantageously in therange of 1:100 to 100:1. In particular embodiments, the ratio of firstto second member is in the range of 1:50 to 50:1.

In embodiments according to the present invention wherein the etchchemistry includes all three members selected from NH₃, NF₃ and N₂O, themole ratio of the first member to the sum of the second and thirdmembers is advantageously in the range of 1:200 to 200:1, and the moleratio of the second member to the third member is in the range of 1:100to 100:1. In particular embodiments, the mole ratio of first to secondand third members is in the range of 1:100 to 100:1, and the mole ratioof second member to third member is 1:50 to 50:1.

A person skilled in the art will recognize that in embodiments accordingto the present invention, the particular choice of non-N₂ memberselected from NH₃, NF₃ and N₂O depends upon the several factors,including the chemical composition of the dielectric to be etched, andthe thickness of the dielectric. Thus, in embodiments according to thepresent invention, the particular non-N₂, nitrogen-containing compoundsselected from NH₃, NF₃ and N₂O that are employed in the etch chemistryand, are advantageously optimized to selectively remove dielectric,while retaining optimal profile characteristics.

As depicted in FIG. 2E, the first photoresist layer 36 is then removedfrom article 30. As discussed more fully above, the first photoresistlayer 36 is subjected to an ash step to remove substantially all of thefirst photoresist layer 36. Comparing FIG. 2E with FIG. IE, it isapparent that the article 10 depicted in FIG. 1E includes a photoresistpoison 18, whereas the article 30 according to the present inventiondepicted in FIG. 2E contains no photoresist poison.

As depicted in FIG. 2F, a second photoresist layer 46 is then applied tothe article 30. The second photoresist may be a suitable photoresist,and may be applied in an art-recognized manner, such as by a spin-ontechnique.

As depicted in FIG. 2G, the second photoresist layer 46 is thenselectively exposed and developed to form a trench pattern aperture 48.As can be seen in FIG. 2G, the trench pattern aperture 48 and via 42 aresubstantially free of poisoned photoresist.

As depicted in FIG. 2H, the dielectric layer 34 is then etched to form atrench 50. The trench 50 is formed by etching the dielectric layer 34using the second photoresist 46 as an etch mask. Then the secondphotoresist 46 is removed, advantageously by an ash method, and thetrench 50 and via 42 are filled with conductive material 52, such ascopper or a copper alloy. The conductive material 52 is etched back toconform to the surface of the dielectric layer 34, for instance bychemical mechanical polishing (CMP) to form the device 30 depicted inFIG. 2H.

The process according to the present invention allows for etching of asubstrate, such as a dielectric comprising SiO₂, with excellent profileand/or selectivity control, while avoiding the deposition of N₂ in thesubstrate. The process according to the present invention thussubstantially avoids the problem of incomplete removal of exposedphotoresist due to photoresist poisoning, especially that poisoningattributable to substrate etching in the presence of N₂. The processaccording to the present invention thereby allows one to carry outmultiple photoresist mask and etch steps with the assurance that secondand later applied photoresist layers will not be poisoned by N₂ leftbehind by earlier etch steps. The process according to the presentinvention thereby provides for more complex, and more accurate,photolithographic methodologies employing etch steps followed by furtherphotolithographic processing.

While this invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not limited to thedisclosed embodiments, but, on the contrary, is intended to covervarious modifications and equivalent arrangements included within thescope of the appended claims.

1. A method of forming an interconnect structure having a first featureand a second feature, comprising the steps of: (a) providing an articlecomprising a conductive layer that has a dielectric layer thereon; (b)depositing a first photoresist layer on the dielectric layer; (c)patterning the first photoresist layer to form a first mask patterndefining the first feature; (d) etching the dielectric layer using thefirst photoresist layer as a mask to form the first feature using anetch chemistry that comprises a member of the group NH₃, NF₃ and N₂O;(e) removing the first photoresist; (f) depositing a second photoresistlayer on the article; (g) patterning the second photoresist layer toform a second mask pattern defining the second feature; (h) etching thedielectric layer using the second photoresist layer as a mask to formthe second feature.
 2. A method according to claim 1, further comprisingdepositing copper or a copper alloy in the first and second feature. 3.A method according to claim 2, wherein the dielectric layer comprisesSiO₂ or a low K dielectric.
 4. A method according to claim 1, whereinthe wherein the etch chemistry comprises NH₃.
 5. A method according toclaim 1, wherein the wherein the etch chemistry comprises NF₃.
 6. Amethod according to claim 1, wherein the etch chemistry comprises N₂O.7. A method according to claim 1, wherein the etch chemistry furthercomprises an inert gas.
 8. A method according to claim 7, wherein theinert gas is selected from the group consisting of He, Ne and Ar.
 9. Amethod according to claim 1, wherein the etchant comprises one or moreof a hydrofluorocarbon, a fluorocarbon, a hydrochlorocarbon, or achlorofluorocarbon.
 10. A dual damascene method of forming aninterconnect structure having a via and a trench, comprising the stepsof: (a) providing a semiconductor wafer comprising a conductive layerthat has a dielectric layer thereon; (b) depositing a first photoresistlayer on the dielectric layer; (c) patterning the first photoresistlayer to form a first mask pattern defining the via; (d) etching thedielectric layer using the first photoresist layer as a mask to form thevia using an etch chemistry that comprises a member of the group NH₃,NF₃ and N₂O; (e) removing the first photoresist; (f) depositing a secondphotoresist layer on the article; (g) patterning the second photoresistlayer to form a second mask pattern defining trench; (h) etching thedielectric layer using the second photoresist layer as a mask to formthe trench; (i) depositing copper or a copper alloy in the trench andvia; and (j) etching the copper or copper alloy back to the surface ofthe dielectric layer.